This invention relates to a distributed feedback (DFB) laser diode or semiconductor laser of a structure which is suitable to manufacture according to metal organic chemical vapor deposition (MO-CVD).
Distributed feedback laser diodes operable in a single longitudinal mode are extensively studied for use in optical communication and in optical measurement. In the manner which will later be described more in detail, such a distributed feedback laser diode comprises a grating semiconductor substrate which has corrugations on its surface. Various semiconductor layers are epitaxially grown on the surface. The semiconductor layers include an active layer and an optical waveguide layer.
Liquid-phase epitaxial growth or epitaxy is usually carried out on growing the semiconductor layers on the semiconductor substrate. It has, however, been unavoidable by the liquid-phase epitaxy that the corrugations are lost or otherwise adversely affected during epitaxy of the semiconductor layers. A method is revealed by the present applicant in U.S. Pat. No. 4,561,915 for an improvement in the liquid-phase epitaxy.
On the other hand, vapor-phase epitaxy is disclosed for the semiconductor lasers by L. D. Westbrook et al in Electronics Letters, Volume 19, No. 11 (May 26th 1983), pages 423 and 424, under the title of "New Approach to the Manufacture of Low-threshold 1.5 .mu.m Distributed Feedback Lasers." The method is called metal organic chemical vapor deposition and is effective in removing the adverse effect which would otherwise be caused to the corrugations. In other words, the metal organic chemical vapor deposition is prosperous in manufacturing distributed feedback laser diodes which are stably operable in a single longitudinal mode, are of excellent characteristics, and are highly reproducible. Laser diodes are, however, unknown which are of a structure suitable to manufacture according to the metal organic chemical vapor deposition.